1. Field of the Invention
The present invention relates to a method for producing a copolymerized polyester resin obtained by copolymerization of a dicarboxylic acid component to impart gas barrier properties.
2. Discussion of Background
Heretofore, an attention has been drawn to polyester resins represented by polyethylene terephthalate resins, in view of excellent mechanical properties and chemical properties, and besides, excellent transparency, gas barrier properties, safety and sanitation and the like. They have been used widely particularly in a food packaging field, as bottles obtained by stretch blow molding of an injection-molded perform, as trays or cups obtained by thermoforming of an extrusion-molded sheet, or as films obtained by biaxial stretching of said sheet.
However, although the gas barrier properties of polyester resins are excellent as compared with e.g. polyolefin resins and polystyrene resins which have been used widely in the food packaging field, the gas barrier properties of the polyester resins may not always be adequate for bottles for e.g. carbonated drinks and alcohol drinks, which require particularly strict barrier properties against e.g. oxygen gas and carbonic acid gas, and such a tendency is marked particularly for miniaturized bottles used in recent years.
On the other hand, for example, as a method to improve gas barrier properties of the polyethylene terephthalate resins, U.S. Pat. No. 4,440,922 (JP-A-60-501060) and U.S. Pat. No. 5,239,045 (JP-A-5-186570) disclose a method of copolymerizing a dicarboxylic acid component containing terephthalic acid or its ester derivative as the main component and a phenylene dioxydiacetic acid as a copolymerizable component, and a diol component containing ethylene glycol as the main component, wherein the phenylene dioxydiacetic acid is preliminarily added at the initiation of an esterification reaction or a transesterification reaction of terephthalic acid or its ester derivative with ethylene glycol for esterification reaction or transesterification reaction, followed by polycondensation, or the phenylene dioxydiacetic acid is added to an esterification reaction product or a transesterification reaction product of terephthalic acid or its ester derivative with ethylene glycol, followed by polycondensation.
However, according to the studies by the present inventors, it has been found that by the above-mentioned methods of adding the phenylene dioxydiacetic acid, the obtained copolymerized polyester resins tend to be slightly yellowed and be poor in color tone, and further, the copolymerization may not always be carried out stably, and accordingly, polyester resins having a stable quality can not be produced.
Further, it has been found that the phenylene dioxydiacetic acid to be used as the copolymerizable component is suscetible to heat deterioration, and it is likely to decompose or turn yellow due to heat history at the esterification reaction or the transesterification reaction or at the polycondensation reaction, and its heat resistance may decrease.
Further, it has been found that foreign substances are likely to be formed in the obtained copolymerized polyester resin, depending upon the time of addition of the phenylene dioxydiacetic acid, and such is problematic particularly as a material to be used in e.g. a food packaging field.